High Resolution Plasma Etch

ABSTRACT

An apparatus for fabrication of microscopic structures that uses a beam process, such as beam-induced decomposition of a precursor, to deposit a mask in a precise pattern and then a selective, plasma beam is applied, comprising the steps of first creating a protective mask upon surface portions of a substrate using a beam process such as an electron beam, focused ion beam (FIB), or laser process, and secondly etching unmasked substrate portions using a selective plasma beam etch process. Optionally, a third step comprising the removal of the protective mask may be performed with a second, materially oppositely selective plasma beam process.

This Application is a Continuation Application of U.S. application Ser.No. 11/766,680, filed Jun. 21, 2007, which is hereby incorporated byreference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to materials processing on a microscopicscale.

BACKGROUND OF THE INVENTION

Modern technology demands the fabrication of ever smaller and moreprecise structures for electronic circuits, optical components, microelectromechanical structures (MEMS), and other applications. Many suchstructures and devices, on the scale of micrometers or nanometers, arefabricated on silicon wafers using photolithographic methods.

A typical photolithography technique involves depositing a thin layer ofa photosensitive material called a “photoresist” onto the surface of asemiconductor substrate such as a silicon wafer, by a process called“casting.” Photolithographic imaging is then used to transfer a desiredpattern, designed on a photolithographic mask, to the photoresist byselective exposure to a radiation source such as light. The photoresistis then chemically developed to remove the radiation exposed areas (in apositive resist) or the unexposed areas (in a negative resist), leavingbehind a pattern of photoresist to protect specific parts of thesubstrate during subsequent processes such as etching (removingmaterial), deposition (adding material to the substrate surface), ordiffusion (diffusing atoms into the substrate). Etching can beperformed, for example, using a reactive chemical, sometimes in the formof a plasma. A plasma can also be used to sputter material from asurface by causing charged particles from the plasma to impact thesurface with sufficient momentum to displace surface molecules.Deposition can be performed, for example, by chemical or physical vapordeposition or plasma enhanced chemical vapor deposition. Afterprocessing, the patterned photoresist is removed. Lithography processare time consuming and, while efficient for processing a complete wafer,are less useful for localized processing.

Focused beams, such as focused ion beams (FIBs), electron beams, andlaser beams, are also used for forming small structures. While beingable to form extremely precise structures, processing by such beams istypically too slow to be used for mass production of fine structures.FIBs can be used to sputter a substrate surface because they employ arelatively large ion such as, for example, a gallium ion (Ga⁺) that canbe accelerated easily to achieve the momentum needed to displacemolecules of the substrate. FIBs can also be used with a precursor gasto enhance etching chemically or to deposit a material onto the surface.Electron beams can also be used, together with an assisting precursorgas, to give rise to etching or deposition processes.

Electron beam, FIB, reactive gasses and plasma processes can be usedeither alone or in combination with one another to manipulate substratesurfaces, for example, to create and repair photolithographic masks.Reactive gasses typically also exhibit material selectivity. Theseprocesses can provide varying degrees of fabrication tolerances,material characteristics, processing times and machining flexibility.

Many problems still exist, however, with the methods of fabrication ascurrently used and described above. For example, it is difficult toprecisely fabricate high aspect ratio holes, that is, holes having adepth that is much greater than their widths. Because currentfabrication processes cause holes or trenches to become wider as theyare etched deeper, adjacent deep features must be spaced further apartthan desired. Time consuming photolithography processes are efficientfor processing entire wafers, but are not useful for processing localregions on a wafer. Conversely, direct-write FIB and electron beaminduced processes are efficient for highly localized processing employedin nano-prototyping, circuit edit, and photolithographic mask repair,but are not useful for processing entire wafers.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method and apparatus forfabricating structures on the micro and nano scales.

In accordance with the foregoing objects, the present invention includescreating a protective coating upon portions of a substrate preferablyusing a focused beam process and then etching unmasked substrateportions using a material selective plasma beam formed using an ionfocusing column positioned between a plasma chamber and the substrate.

Finally, many other features, objects and advantages of the presentinvention will be apparent to those of ordinary skill in the relevantarts, especially in light of the foregoing discussions and the followingdrawings, exemplary detailed description and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Although the scope of the present invention is much broader than anyparticular embodiment, a detailed description of the preferredembodiment follows together with illustrative figures, wherein likereference numerals refer to like components, and wherein:

FIG. 1 is a flowchart showing the preferred order of processing of thepresent invention.

FIG. 2 shows an elevated side view of a substrate sample having an uppersurface;

FIG. 3 shows the substrate of FIG. 2 further showing portions of aprotective mask upon the surface;

FIG. 4 shows a collimated plasma beam selectively etching an unprotectedportion of the substrate of FIGS. 2 and 3;

FIG. 5 shows schematically a dual beam system comprising an electroncolumn, and the combination of a plasma chamber and a differentiallypumped ion focusing column for the generation of a plasma beam

FIG. 6 shows the selective etching of the protective mask of FIGS. 2 and3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Although those of ordinary skill in the art will readily recognize manyalternative embodiments, especially in light of the examples providedherein, this detailed description is exemplary of a preferred embodimentof the present invention, the scope of which is limited only by theclaims appended hereto.

Preferred embodiments of the invention relate to a method forfabricating nanoscopic and microscopic structures, such as integratedcircuits or micro-electromechanical systems, by a beam used to locallyfabricate a protective mask, followed by use of a plasma beam etchprocess.

Where lithography is typically used for pattern definition inmicromachining, beam processes as used in preferred embodiments of thepresent invention create precisely defined protective mask patterns on asubstrate surface and, when followed by etching using a plasma beam,contribute to superior structural tolerances and machining flexibility.For example, beam deposition can produce surface features such as linewidths of less than about 10 nm.

Referring now to the FIGS, FIG. 1 is a flowchart that shows a preferredorder of steps of a method of the present invention. FIGS. 2 through 4and FIG. 6 illustrate the various stages of processing of a substrate inaccordance with the method shown in FIG. 1. FIG. 2 shows a samplesubstrate 200 having a surface 202 before processing begins.

In step 102, a focused beam (e.g., with a beam diameter in the range of1 nm to 100 μm, and more commonly in the range of 1 nm to 10 nm) is usedto fabricate a protective layer to mask a portion of the substratesurface to protect the masked portion during a subsequent etching step.For example, the beam can be used to deposit a masking material usingbeam-assisted deposition, or the beam can be used to etch a pattern in alayer of mask material previously deposited by other known methods. Inone preferred embodiment, the beam is used to deposit a material thatwill coordinate with the subsequent etching step insofar the depositedmaterial is etched less by the subsequent process than the exposed orun-masked surface of the substrate material. Beam processes such asthose used in preferred step 102 typically produce higher definitionprotective layers than are available through photolithographic means.Furthermore, beam processes are able to process arbitrary regions of asubstrate immediately after inspection using the focused beam, withoutthe need to fabricate conventional photolithographic masks. This makesbeam processes particularly useful for one-off modifications such asthose encountered in photolithographic mask repair, circuit edit, andnanostructure prototyping. Beam processes are, in general, especiallysuited to localized deposition, by which is meant deposition on aportion of the substrate that is significantly smaller than the entiresubstrate.

FIG. 3 shows using a beam 302 to deposit a protective layer or mask 304upon one or more portions 306 of the surface 202, leaving behind anunmasked region 308 to be processed by a plasma beam as will be betterunderstood further herein. In a preferred embodiment, beam 302 comprisesa charged particle beam, such as an electron beam. A method for electronbeam deposition is described, for example, in U.S. Pat. No. 6,753,538 toMusil, et al. entitled “Electron Beam Processing.” Musil describes usingan electron beam to deposit material by inducing a chemical reaction ina precursor gas that decomposes in the presence of the electron beam toleave an appropriate material on a surface. Preferable depositionprecursor gasses include, for example, styrene (C₆H₅CH=CH₂) fordepositing a carbon protective mask that is removed slowly by XeF₂ etchprocesses, and WF₆ (tungsten hexafluoride) or W(CO)₆ (tungstenhexacarbonyl) for depositing a tungsten-rich protective mask that isresistant to oxygen-based carbon etch precursors such as H₂O. Precursorgases that result in the deposition of an oxide layer include, forexample, TEOS (tetraethylorthosilicate), which deposits a silicon oxideprotective mask. Other precursors for electron beam deposition are knownand can be used as well. Examples include perdeuterated gallium azide(D₂GaN₃) and Pt(PF₃)₄ (tetrakis (trifluorophosphane) platinum) for thedeposition of masks rich in GaN and Pt, respectively.

Those of ordinary skill in the art of charged particle beam depositionwill appreciate that deposition can be done across a range of electronbeam energies and currents, depending on the deposition rate andresolution required. Generally, high beam current and low beam energyresult in higher deposition rates, with an optimum beam energy of about100 eV. Energies below this value typically result in lower depositionrates. However, for optimum resolution of the deposit, higher beamenergies are typically required to give a smaller electron beam.Therefore, energies might reasonably range from about 1 keV to about 30keV with conventional scanning electron microscopes (SEMs) such as,those available from FEI Company, the assignee of the present invention.Energies as high as 300 keV, typically used in transmission electronmicroscopes (TEMs), can also be used.

Preferred beam currents range from picoamps to nanoamps. However, therecould be applications, for example, in some MEMS fabrication, where muchhigher currents in the microamp range are preferred when very highdeposition rates are required and low resolution is acceptable. Typicalelectron beam spot sizes can range from 1 nm to 100 nm, while spot sizesranging from 0.1 nm to 10 μm may be useful in some applications,spanning TEM systems and very high electron current SEM columns.

Alternatively, the beam 302 used to deposit material forming theprotective mask 304 may be a FIB. In a FIB process used for deposition,a gas is directed toward a surface and a finely focused ion beam(typically comprising gallium ions), decomposes the gas moleculesabsorbed on the target surface and causes the metal products to becomedeposited. A process for depositing metal material using a FIB isdescribed, U.S. Pat. No. 4,609,809 to Yamaguchi, et al. for “Method AndApparatus For Correcting Delicate Wiring of IC Device,” which describesthe use of a tungsten precursor gas compound in the presence of the FIBto deposit tungsten upon a substrate surface. Other precursor gassesthat decompose in the presence of a FIB to deposit a material and can beused with the invention are described, for example, in Handbook ofCharged Particle Beam Optics, Ed. Jon Orloff, CRC Press (1997). The ionbeam can be focused to a point or shaped, as described for example, inU.S. Pat. No. 6,977,386 for “Angular Aperture Shaped Beam System andMethod” to Gerlach et al., which patent is assigned to the assignee ofthe present invention. The tern “focused beam” as used herein includes ashaped beam.

Laser-beam-induced deposition can also be used. The beam can depositmaterial by providing energy to decompose a precursor as describedabove, or the beam can be comprised include particles, such asfullerenes, that are deposited onto the surface, as described, forexample in U.S. patent application Ser. No. 11/590,570 for“Charged-Particle Beam Processing Using a Cluster Source.”

In any case, the methods of deposition, and the material deposited, arepreferably such that the resulting protective mask 304 can beselectively removed in a subsequent mask-removal step, as will bedescribed below. As described above, the mask can also be created bycoating the area of interest with a layer of masking material, and thenusing the beam to remove material from the masking layer to form adesired pattern.

FIG. 1 shows that the second step 104 comprises directing a firstcollimated plasma beam toward the substrate surface, the beamselectively etching the substrate surface that is not covered by themask at a significantly higher rate than it etches the protective mask.Preferably, the first plasma beam 402 exhibits a selectivecharacteristic so that the unmasked material is preferably etched at arate that is at least two times greater than the etch rate of theprotective mask 304 material, more preferably five times greater, evenmore preferably ten times greater, and most preferably one hundred timesgreater.

Referring to FIG. 4, a first collimated plasma beam 402 is directedtoward the surface 202 of the substrate sample 200 to etch the un-masked(un-protected) region. The preferred degree of collimation depends onthe desired properties of the etched feature and the desired size of thebeam. A more highly collimated beam produced more vertical sidewalls inthe etched feature. The degree of collimation also affects the beam spotsize. The first plasma beam 402 is preferably collimated andsubstantially unfocused, such that the beam spot area is preferablygreater than the unmasked area of the substrate surface, but smallerthan the area of the mask, That is, the plasma beam is preferablysufficiently broad to etch the entire unmasked region 308, although theplasma beam or sample can be moved to scan and etch all required regionsin some embodiments. The plasma beam preferably has a constant ion fluxprofile across the entire unmasked etch region 308 to etch the areaevenly. The ion flux is preferably constant to within about 20% acrossthe unmasked area of the substrate surface, more preferably to withinabout 1%, even more preferably to within about 0.1% and most evenpreferably to within about 0.001%. Plasma beam 402 is local, that is, itcovers an area that is significantly smaller that the entire sample.When used to etch high aspect ratio holes, plasma beam 402 is preferablyhighly collimated to avoid tapering the etching of the holes. In formingsome features, such as low aspect ratio holes, collimation is lessimportant.

The first plasma beam 402 preferably has sufficiently high energy tocause ions in the plasma beam to dissociate upon contact with thesurface 202, and sufficiently low energy to prevent significantsputtering of the surface 202. That is, rather than material beingremoved by momentum transfer from the ions of the plasma beam to thesurface, material in unmasked region 308 is removed primarily by achemical reaction between a reactive molecule formed by the dissociationof ions in the plasma beam upon contact with the sample surface, thechemical reaction forming a volatile byproduct that is evacuated fromthe vacuum chamber. The energy of the ionized particles is preferablysuch that the material removal rate caused by sputtering is at leastfive times lower than that caused by chemical etching, preferably 10times lower, more preferably 100 times lower, and most preferably 1000times lower. The selectivity is such that the mask is preferably notcompletely removed when the etching process is completed, or at leastthe mask is in place long enough that the masked area is subject to anacceptable amount of etching. The combination of ion dissociation andsputter prevention is achieved by fine-tuning the ion landing energy atthe surface. Typical energies lie in the range of 1 eV and 10 keV, andmore preferably in the range of 10 eV and 500 eV. The ions from theplasma disassociate on contact with the surface 202 and etch theprotective mask 304 little or not at all, while etching primarily theun-masked region 308.

Reactive plasma precursors that may be used include, for example, SF₆and XeF₂, which can be used to form plasma beams composed of, forexample, SF₆ ⁺ and XeF₂ ⁺, respectively.

FIG. 4 shows plasma beam 402 in the process of forming a trench 410having sidewalls 412. It will be appreciated by those of ordinary skillin the art that the plasma etch process described above will tend toetch high aspect ratio trenches with significantly parallel sidewalls.This is so because (i) the plasma beam is collimated, (ii) the volatilebyproducts do not re-deposit onto the pit sidewalls, and (iii) neutralgas molecules are reduced or eliminated inside the chamber containingthe sample 200 by having the sample 200 positioned outside the plasmachamber. The last point is significant because the plasma beam givesrise to secondary electron emission from the sample. Said electrons canimpact sidewalls 412, where they can dissociate adsorbed etch precursorgas molecules, giving rise to (typically undesired) lateral etching ofthe sidewalls. Such lateral etching is absent when neutral etchprecursor molecules are not present inside the specimen chamber. In anideal plasma source, only ionized gas molecules are extracted and usedto form a plasma beam. In practice, neutral gas molecules diffuse intothe plasma beam focusing column and the specimen chamber. In a preferredembodiment of the invention, the neutral molecules can be prevented fromentering the specimen chamber by the employment of a differentiallypumped plasma focusing column.

FIG. 5 shows schematically a dual-beam system 500 implementation of thepresent invention. System 500 includes an ion beam column 502 and anelectron beam column 504. While ion beam column 502 is shown verticallyoriented and electron beam column 504 is shown oriented at an angle tovertical, other embodiments may employ a vertical electron beam column504 and a tilted ion beam column 502. Ion beam column 502 includes aplasma chamber 504 which is supplied with a gas or gasses through a gasinlet 506. Plasma chamber 504 can use, for example, an inductivelycoupled, magnetically enhanced plasma source to generate a plasma.Ionized gas molecules and some neutral gas molecules leave plasmachamber 504 through an aperture 508. Ions are accelerated toward a workpiece 510 positioned on a positionable stage 512. A series of apertures520 allow differential vacuum pumping along ion column 502, so that mostof the neutral molecules that leave plasma chamber 504 can be removedfrom the ion column by various gas outlets 522 along column 502.

Ion column 502 includes a beam blanker 530 for blanking the ion beam andbeam deflectors 532 for positioning the beam on the work piece 510 andscanning the beam. A lens 534 collimates or focuses the beam of ionizedgas molecules. Unlike a typical semiconductor plasma processing system,in which the work piece is positioned in the plasma chamber, embodimentsof the present invention separate the plasma chamber 504 from the workpiece 510. This separation provides an opportunity to collimate or focusthe beam as it travels through ion optical elements in the ion column,remove most of the neutral molecules thereby preventing them fromreaching the work piece, and control the ion beam impact energy at thework piece.

It will be appreciated by those of ordinary skill in the art that theplasma etch process described above will be able to achieve very highmaterial removal rates and very high lateral process resolution. Thematerial removal rate is determined by the plasma beam current, and isnot limited by adsorbate depletion in the surface region irradiated bythe beam, a well known effect that often limits the process rates ofgas-assisted charged particle beam etch and deposition processes.

Other attributes of the first plasma beam 402 are a comparatively largerbeam diameter than one of a typical liquid metal ion source FIB, butsmaller compared to a typical inductively coupled plasma (“ICP”)reactor. It is contemplated that the beam diameter will range in sizefrom several tens of nanometers to several millimeters. That is, theplasma beam is typically at least an order of magnitude and morepreferably two or more orders of magnitude, greater than the diameter ofthe beam used to produce the protective layer. Thus, the beam thatpatterns the protective layer can create a complex pattern of features,and the plasma beam can encompass the entire pattern to etch the patternin one step. The plasma beam diameter is typically less than one half ofthe substrate diameter, and more preferably less than one tenth of thesubstrate diameters. Thus, the process is typically used as a localizedetching process, and not an entire wafer process. The current of thefirst plasma beam 402 may range from a few picoamps to severalmilliamps, but typical beam currents are more likely to range from 1nanoamp to a few microamps.

For this process, a magnetically enhanced, inductively-coupled plasmasource is preferred, as the high brightness of this source isparticularly adept. Such an ICP source is described in U.S. Pat. App.Pub. 2005/0183667, to Keller et al. for “Magnetically Enhanced,Inductively Coupled Plasma Source for a Focused Ion Beam System.” One ofordinary skill in the art will appreciate that the higher the brightnessof the source, the more collimated the beam will be for a given beamcurrent and, therefore, the more precise the etching will be. However,any other plasma ion source could also be used for this step such as,for example, a duoplasmatron, penning ion source or a capacitivelycoupled plasma source.

Reactive plasma precursors that may be used include, for example, SF6and XeF₂. One preferred embodiment contemplates the use of XeF₂ in thesecond step when the protective layer was formed from tungsten due tothe selectivity of XeF₂ for etching silicon over tungsten.

A non-reactive plasma gas can also be used together with an etchantprecursor gas that is introduced into the vacuum chamber by a gasinjection system 540 having a nozzle 542 positioned near the work piece510. In such a case, rather than the molecules of the plasma reactingwith the sample, the plasma molecules and secondary electrons emitted asa result of ion impact at the sample provide the energy to induce areaction between the sample and an etch precursor that is introducedinto the vacuum chamber near the sample. In some embodiments, argon canbe used as in the plasma because it has a low ionization potential and,therefore, requires less energy to break down and sustain as a plasmabeam. The argon can be used with a precursor gas, or the argon can bemixed with a reactive gas in the plasma chamber. In any case, it ispreferable that the plasma beam and/or the combination of the plasmabeam and an assisting gas be chosen to exhibit selectivity in this stepto etch the exposed un-masked region 308 and not the protective mask304.

In a third step 106, the masking material is removed. Referring to FIG.6, a second plasma beam 602 is directed toward the sample. In someembodiments, the beam is sufficiently large that it can remove the maskwithout scanning. Plasma beam 602 is selective for etching material ofthe protective mask 304 while not etching, or etching at a significantlylesser rate, the substrate sample 200. The mask material is preferablyetched at a rate that is at least two times greater than the etch rateof the substrate material, more preferably at least five times greater,even more preferably at least ten times greater, and most preferably atleast one hundred times greater.

For example, an oxygen ion plasma beam can be used to volatilize acarbonaceous mask, and a XeF₂ plasma beam can be used to remove atungsten mask. In other embodiments, a number of gases can be mixed toproduce a plasma beam with specific etch characteristics. For example,oxygen can be mixed with SF₆ in the plasma to enhance the etch rate ofsome substrates. For example, a mixture of SF₆ and 5% oxygen reduces theetch rate of silicon as compared to pure SF₆, but still etches metalssuch as tungsten very quickly. This gas mixture gives good selectiveetching of tungsten when the exposed substrate is SiO₂ at, or about, theration of 10:1 for W:SiO₂. Alternatively, a pure oxygen plasma beam canbe used to initially oxidize the entire surface, followed by theabove-mentioned use of SF₆ and 5% oxygen. In other embodiments, theprotective layer can also be removed by any convenient method, includingfocused ion beam milling, a selective chemical etching process orchemical mechanical polishing. The diameter of beam 602 in FIG. 6 issmaller than the mask area, so the beam 602 is scanned over the maskedarea.

In other embodiments of the invention, the mask may be removed by astandard focused ion beam sputter process (such as that employed in Galiquid metal ion source FIB tools), or by a combination of chemicaletching and physical sputtering. We note that such processes will causean “over-etch” the unmasked region, but the over-etch will be negligibleif, after step 104, the mask thickness is small relative to the depth ofthe etch pits produced in step 104.

While the foregoing description is exemplary of the preferred embodimentof the present invention, those of ordinary skill in the relevant artswill recognize the many variations, alterations, modifications,substitutions and the like as are readily possible, especially in lightof this description, the accompanying drawings and claims drawn thereto.For example, the trench 410 can be filled by a gas-mediated chargedparticle beam-induced deposition process before or after the maskingmaterial 304 is removed in step 106. In any case, because the scope ofthe present invention is much broader than any particular embodiment,the foregoing detailed description should not be construed as alimitation of the scope of the present invention, which is limited onlyby the claims appended hereto.

We claims as follows:

1. An apparatus for etching a sample substrate to form microscopicstructures, comprising; a gas injection system; a first system includinga beam generating column for generating a fine beam, the gas injectionsystem positioned to direct a gas toward the sample near the impactpoint of the beam to deposit a masking material in a pattern on thesurface; a second system including: a plasma chamber for generatingions; an ion optical column for receiving ions from the plasma chamber,the ion column including one or more lenses for forming an ion beam byfocusing or collimating the ions from the plasma chamber into a beam;and one or more accelerating electrodes, the accelerating electrodebeing maintained at a voltage to provide the ions with an energysufficient to react chemically with the unmasked surface butinsufficient to sputter the surface.
 2. The apparatus of claim 1 inwhich the first system comprises an electron beam system or a focusedion beam system.
 3. The apparatus of claim 1 in which the first systemcomprises a laser system.
 4. The apparatus of claim 1 in which thesecond system comprises an inductively coupled plasma system.
 5. Theapparatus of claim 1 in which the second system produces a beam having adiameter at least an order of magnitude greater than the diameter of thebeam from the first system.
 6. The apparatus of claim 1 in which thesecond system produces a beam having a diameter less than one half thesubstrate diameter.
 7. The apparatus of claim 1 in which the secondsystem produces a beam having a diameter less than one tenth thesubstrate diameter.
 8. The apparatus of claim 1 in which the secondsystem produces a beam having a current of between a few picoamps andseveral milliamps.
 9. The apparatus of claim 1 in which the secondsystem produces a beam having a current of between one nanoamp and a fewmicroamps.
 10. The apparatus of claim 1 in which the gas injectionsystem comprises a source of a tungsten precursor and in which thesecond system comprises a source of SF₆ or XeF₂ for the plasma chamber.11. The apparatus of claim 1 in which the second system furthercomprises a gas source for the plasma chamber, the gas source providinga gas that, after leaving the plasma chamber, will comprise reactiveions that will react chemically with the unmasked surface.
 12. Theapparatus of claim 1 in which the plasma chamber provides ions of aninert gas to the focusing column and in which the gas injection systemprovides an etchant precursor that is activated by the ions of the inertgas.
 13. The apparatus of claim 1 in which the second system comprisesdifferentially pumped chambers between the plasma source and thesubstrate to remove neutral gas molecules.
 14. The apparatus of claim 1in which the substrate is positioned outside of the plasma chamber. 15.The apparatus of claim 1 in which the one or more lenses of the ioncolumn provides a collimate beam.
 16. The apparatus of claim 1 in whichthe one or more lenses of the ion column provides a convergent beam. 17.The apparatus of claim 1 in which the second system provides ionizedparticles at the sample having an energy in the range of 10 eV and 500eV.
 18. The apparatus of claim 1 in which the second system provides asubstantially uniform ion flux profile across the unmasked region of thesubstrate surface.
 19. The apparatus of claim 1 in which the secondsystem provides a beam that selectively etches the protective maskmaterial more rapidly than the sample substrate material.
 20. Theapparatus of claim 1 in which the second system provides a collimatedbeam of ions, the average energy of the ions being less than 500 eV.